1. Field of the Invention
The present invention is directed operating a probe-based instrument in torsional oscillation mode, and more particularly, a method and apparatus of driving the probe into torsional resonance.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip and low forces to characterize the surface of a sample down to atomic dimensions. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the tip of the associated probe assembly. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “scanning probe microscopy.”
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc.
Preferably, the probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant by maintaining constant deflection of the cantilever. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation “flexural mode” of operation in which the cantilever oscillates generally about a fixed end. One popular flexure mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the cantilever probe is oscillated flexurally at or near one of its resonant frequencies. When the tip is in intermittent or proximate contact with surfaces the oscillation amplitude will be determined by tip/surface interactions. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Independent of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers typically fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
One limiting characteristic of AFMs and other probe-based instruments lies in the above-described modes of operation. In an AFM, the cantilever is typically oscillated using a piezoelectric drive, often known simply as a piezo drive, to provide, for example, a flexural oscillation mode. Referring to FIG. 1, a probe assembly 20 includes a probe 21 having a cantilever 22 and a tip 28. The cantilever 22 extends outwardly from a base 24 of assembly 20. The cantilever 22 may be attached to the base 24 or formed integrally with it. Base 24 is typically coupled to a piezoelectric drive 26 (e.g., a piezo stack). Tip 28 is provided on the opposed, free end of cantilever 22. Piezoelectric drive 26 can be selectively excited by a signal generator 30 to move cantilever 22 up and down relative to a sample 32. When the instrument is configured for flexural oscillation mode operation, the drive voltage is applied to piezoelectric drive 26 to flexurally oscillate the cantilever 22 about a lateral axis of the probe 21 at a frequency that is dependent upon the frequency of the drive voltage.
More particularly, in flexural oscillation mode, cantilever 22 is driven to resonate at its flexural resonance frequency or a harmonic thereof about a lateral axis A–A′ at the base 24 of cantilever 22. Characteristics of cantilever flexural oscillation, and changes thereof, are detected by quadrature photodetector 34, typically with its vertical components, as shown by the arrow “V” in FIG. 1. The deflection angle is sensed by photodetector 34 and output as a voltage signal. Notably, the amplitude of the flexural oscillation ranges between a few nm to 100 nm peak-to-peak depending on the cantilever length.
In operation, as tip 28 approaches a surface of sample 32, the flexural oscillation (tapping) amplitude starts to decrease due to contact between tip 28 and sample 32. Notably, the flexural vibration amplitude decreases to zero when tip 28 is pushed against sample 32 with constant contact pressure. Variation of amplitude between zero (generally continuous contact) and free oscillation is typically used in a feedback configuration to control tip/surface distance. Alternatively, the phase of the flexural oscillation may be used to control this distance. Information relating to the surface such as topology, hardness, and/or electromagnetic properties is then determined by analyzing the signals that are used to control this tip/surface spacing.
Overall, flexural oscillation mode AFMs are used to characterize surface topology and surface energy dissipation by monitoring the amplitude and/or phase of the oscillating cantilever. This mode is often preferred to contact mode imaging because it produces less damage to the tip and sample during operation. However, operating the AFM based on flexural oscillation of the cantilever is constrained in the following aspects.
Initially, flexural mode operation only detects surface characteristics that impart a force in one direction, namely, the vertical or “Z” direction. As a result, flexural mode AFMs do not detect shear force interaction, and thus also cannot provide shear force or force gradient information. This information is critical to making measurements of surface friction, for example, when attempting to identify surface compositional differences. When the topography of the materials is generally undifferentiated, minimal information is provided by flexural mode operation, and thus this friction information becomes particularly valuable, and sometimes necessary. Applications include identifying different components in polymer blends, composites and other mixtures, identifying organic and other contaminants on sample surfaces, delineating coverage by deposited coatings and other surface layers, etc.
Moreover, without shear force or shear force gradient measurement capabilities, flexural mode operation often results in loss of other information relating to the sample. For example, when a flexural oscillation mode AFM is used to image the magnetic domain of a sample, only a force gradient in the direction perpendicular to the sample surface can be sensed. Domains parallel to the surface can only be seen at the domain boundaries where the transitional region has a vertical force gradient. This limitation also holds true for electric force imaging.
Other drawbacks associated with flexural resonance imaging are slow kinetics and small amplitude errors that can drastically limit scanning and data acquisition speed and compromise image integrity. This effect is illustrated in the response curve 40 of FIG. 2. In this case, Ao is the free air amplitude of oscillation (in RMS voltage), and As is the set-point amplitude for the control loop. When Ao starts to decrease from a constant value, Ao, the tip starts to tap on the sample surface. When tip/sample separation is reduced, and the tip and sample interact, there is a corresponding change in the signal produced by the deflection detection system. The amplitude of flexural oscillation of the lever decreases due to it being constrained by the sample surface as the tip approaches the surface and taps the sample in each stroke of the oscillation. This is shown in region “O” in which tip-surface distance (x-axis) is smaller than half of the peak-to-peak oscillation of the cantilever. Notably, a feedback loop operates to move the cantilever up and down to keep generally the same oscillation amplitude As. This movement reflects height changes in the sample, i.e., surface topography.
The response of the cantilever in this flexural mode is illustrated by the slope of the curve at region “O.” In other words, for a particular change in tip/sample separation (ΔZ1), the corresponding measured change in voltage (ΔVf) is relatively small given the shallow slope. It is this measured change that determines the error that is processed by the feedback loop to return operation to the set-point oscillation. Because the slope of the cantilever response in flexural mode is relatively shallow, scan speed must be kept small as relatively large changes in tip-sample separation produce a relatively small change in measured output, or error. Therefore, to facilitate adequate data collection and integration of error signals, the scan time at each location (or image pixel) must be long enough for the system to respond with accuracy and resolution. The speed of data acquisition must be correspondingly limited as well. An improvement in data acquisition speed was desired.
Moreover, the shallow slope of the amplitude/distance curve in FIG. 2 makes the control signal (voltage in the vertical axis) correspond to a large height or distance compared to an amplitude/distance curve with a steeper slope. As a result, the control error will correspond to a greater quantity of height measurement error. The situation is particularly problematic when the probe is scanning across an abrupt step where slower response due to error integration will result in even greater inaccuracy for a given scan speed. Notably, such inaccuracy may be detrimental to obtaining useful data in semiconductor metrology. The response of an improved AFM, according to the present preferred embodiments (AFM operating torsional resonance mode), is illustrated in FIG. 2A and will described in further detail below.
Yet another limitation with flexural mode operation is that the flexural resonance is very sensitive to the imaging environment (e.g., when the sample is immersed in water), and thus oscillation properties often change drastically, and in unpredictable ways, upon change in imaging environment. Currently, the sensitivity of flexural mode operation to imaging environment is one of the most significant design considerations when configuring an AFM for operation in fluid.
Other modes of AFM operation are similarly limited. For example, shear force interaction between the probe in contact mode and the corresponding sample surface has been studied with AFM for a number of years. In an AFM technique known as lateral force microscopy (LFM), the cantilever tip is dragged across the sample surface, as in contact mode, to measure friction forces, as described in U.S. Pat. No. 5,553,487 to the present assignee. More particularly, using LFM, the tip is introduced to the sample surface under a constant flexural deflection and then scanned along the surface either in the direction of the cantilever length, or perpendicular to the cantilever length. Using a laser-based deflection detection system, the lateral cells of the corresponding photodetector sense rotation of the cantilever as the tip of the probe interacts with the sample through friction force. In the case where tip scanning direction is perpendicular to the cantilever, the difference of the lateral deflection during forward and reverse scanning of the same portion of the sample is used as a relative measure of the shear force, or surface friction. In addition to the drawbacks associated with using contact mode to detect topology characteristics, including tip/sample damage, etc., LFM suffers the disadvantage of large tip/sample forces associated with contact mode, and poor repeatability.
In other techniques, the tip placed in contact with the sample surface is modulated by moving the sample surface laterally relative to the probe. In this case, the lateral rocking of the cantilever as a result of the contact friction is used to indicate a quantity of surface friction. However, the lateral deflection signals are small, and thus often unusable, and resolution is insufficient for some of the applications contemplated by the present invention.
In addition, although lateral deflection signals induced by motion of the sample at acoustic frequencies can be enhanced, the main control loop that defines tip/surface relative position still employs vertical deflection (contact mode) feedback and, therefore, suffers the drawbacks associated with flexural contact modes.
One challenge in implementing an AFM to image in torsional resonance mode is that optimum performance depends on efficiently driving the cantilever probe into torsional resonance. It is important to note that typical AFM cantilever probes are manufactured for flexural motion, i.e., motion in the “Z” direction. This is primarily due to the fact that the optical system used in detecting mechanical changes in the oscillating motion of a cantilever are aligned in the “Z” direction. Also in this regard, probes designed for flexural motion are easy to produce in batches while preserving their planarity, which is important to ensure accurate measurement of changes in flexural oscillation.
A potential problem arises with standard AFM cantilevers designed for flexural oscillation due to the fact that there is physical asymmetry along the corresponding length of the lever which although acceptable when driving the probe into flexural oscillation, can render driving the cantilever into pure torsional resonance difficult. For instance, if torsional resonance is excited with two piezoelectric actuators driven out of phase, the piezo-actuators must be disposed generally symmetrically about (1) the length of the cantilever, along its central axis, and the (2) corresponding tip to create pure rotational motion of the cantilever about a rotation center, approximately equidistant from the two piezoelectric actuators. The problem is, it is difficult to insure that the central axis of the probe lies at the rotation center of the drive. For instance, mounting the probe (i.e., probe chip) in the AFM head is often an imprecise task due to allowable variations in probe chip position, as well as structural variations of the probe chip and probe itself. In addition, even if the actuators are positioned equidistant from the central axis of the cantilever, due to fabrication limitations, it is difficult to determine whether the drive actuators are positioned symmetrically about the tip which, although preferably resides at the central axis of the lever, often does not.
Notably, in this regard, the tip of the probe attached to the lever is relatively massive (it can be as much as fifteen microns long) such that the inertia of the tip causes a torque. This torque produces an arcing motion at the apex of the tip, but given the scale that this motion occurs, the apex moves substantially horizontally. Overall, however, it is difficult to determine the amount of lateral motion that can be provided by such a system due to inefficiencies coupling the energy to the tip caused by the imperfect spatial relationship between the probe tip and the drive.
As suggested previously, if the probe is well centered between the two piezo actuators (eg., plates) that are employed to drive the probe into torsional resonance, then the system will excite pure lateral motion of the tip. As a practical matter, however, because the tip typically is not centered, due to, for example, imperfections produced during the probe manufacturing process which may cause the tip to be positioned off the central axis of the cantilever or the entire probe to be mounted off-center intermediate the two piezo plates when secured in the AFM head.
Whatever the cause, this compromised relationship between the drive and the probe can produce imperfect lateral motion of the cantilever, and thus the tip. In particular, often times, this motion will exhibit a vertical component. Such non-ideal motion lowers the efficiency of operation in torsional resonance mode. For instance, a vertical component in the cantilever motion can make maintaining operation at the setpoint difficult. Moreover, due to the high “Q” associated with torsional resonance mode, the overture of the flexural vibration of the cantilever is fairly close to the fundamental torsional resonance oscillation frequency of the cantilever. Therefore, the vertical component of cantilever motion may become mixed with torsional oscillation such that the system becomes very unstable, with the possibility that AFM operation toggles between, for instance, torsional resonance and flexural resonance modes of AFM operation. Clearly, this unpredictability is non-ideal. Overall, due to the many potential pitfalls with insuring true alignment between the probe tip and the center of rotation produced by the drive, including imperfect mounting of the probe within the AFM had, an alternative driving arrangement was desired.
Maintaining oscillation generally at the true torsional resonance of the probe is particularly important when considering the range of applications offered by operating the AFM in torsional resonance mode. In an AFM application that is particularly interesting, the probe is used to manipulate, for instance, nanoparticles. Given the scale of operation, one key challenge in using an AFM probe to perform nanomanipulation is determining whether a particular operation associated with manipulating nanoparticles has actually been accomplished. In standard AFM operation, once a target to be manipulated has been identified and an operation attempted by the AFM probe, there is no convenient way to determine whether the target has actually been acted on. For instance, if the operation is to pick up a target, one might think that the change in weight at the tip could be measured. However, because the target to be manipulated typically does not have an appreciably greater weight than the tip, methods based on directly measuring a change in weight are unreliable and, in any event, difficult to implement. In fact, there is such a small change in mass at the tip, e.g., one part in a million of the entire cantilever, directly measuring the change is generally impossible. As such, an alternate technique, preferably one which observes a unique parameter associated with the AFM probe, was therefore desired.
In addition, known techniques for performing nanomanipulation, such as ones that employ what are known as “nanotweezers,” have significant limitations. Most such techniques only have the ability to manipulate targets that are on the scale of a micron or even a little larger. This is due to the fact that there are often difficulties associated with locating the tweezers at a location of interest, and controlling the force applied by the tweezers to the sample, for example. In one technique, electrostatic forces are used to actuate two adjacent columns or beams. In this case, a voltage is applied to at least one of the beams to modify the attractive force between the two beams, thus causing the beams to close one against the other. By controlling the voltage, the beams can be used to close on an object to be manipulated. One problem with such a system is that large magnitude forces are required to close the arms, which can compromise the object being manipulated. Moreover, in this regard, the dimensions of candidate target objects to be manipulated are correspondingly limited in that objects that are too small cannot be feasibly manipulated, especially given the higher voltages, and thus the higher forces, that are difficult to accurately control. Development of manipulation applications is continuing on the nanoscale, and thus a superior design was desired. In particular, an improved nanomanipulation device with the ability to close a gap of about fifty to two hundred nanometers in a highly controllable and accurate fashion, would be particularly valuable.